An antenna structure is often configured in such a way that it has an asymmetrical structure. In this case, a radiator, such as an electrical monopole, for example, is connected with respect to a ground connection. The antenna structure is usually matched to an output resistance of a transmitting stage of 50 ohms or an input resistance of a receiving stage of 50 ohms. Since, particularly in integrated circuits, the transmitting stage generally has a differential—that is to say symmetrical—output, it is necessary to connect at least one transformer between the asymmetrical antenna structure and the transmitting stage. The transformer converts the output signals of the transmitting stage, so that these can be fed to the antenna structure. Connecting a transformer into the transmitting path causes undesirable power losses, however. A corresponding measure in the form of a transformer between the receiving stage and the antenna structure is provided in the receiving path since the receiving stage quite generally likewise has differential and thus symmetrical inputs.
In order to suppress an emission of undesirable harmonics by the transmitting stage, a filter is inserted between the transmitting stage and the antenna structure. Said filter usually has a high-pass, low-pass or bandpass behavior. Inserting the filter is necessary in order to emit an as far as possible monofrequency and low-noise signal. A filter is equally present in the receiving path between the antenna structure and the receiving stage, which filter reduces undesirable out-of-band signals from the reception signal which have an interfering effect on a preamplifier and mixer. The presence of the filters is associated with the disadvantage of undesirable power losses. These are referred to as insertion losses.
A further disadvantage arises in the case of a DC supply of the transmitting stage via the center tap of a symmetrical transformer. Such a transformer is configured in such a way as to effect a transformation to the asymmetrically embodied antenna structure. The transformer can thus be connected in parallel with the antenna structure. With an additional external capacitor, it is possible in this way to effect a filter blocking effect outside the useful band. A known arrangement of a transmitting stage with differential outputs, an asymmetrical antenna structure and a transformer is known in Bluetooth transceivers or similar transmission devices; the Bluetooth transceiver PMB8761 from the company Infineon shall be mentioned by way of example. The transformer is a wire-wound ferrite element associated with high production costs. Due to the poor material properties of the ferrites at high frequencies, i.e. at frequencies in the range of more than 1 GHz, the losses of the transformers usually increase considerably, which often leads to considerable insertion losses in the transmitting and/or receiving path. This causes a significant impairment of the performance of the overall system.